CONJUGALES loi 



and force the cells apart whilst the rings of the other cell evaginate 

 after separation {replicate fragmentation) (cf. fig. 67 F-H). 



(c) The septum develops an I piece and then when the wall 

 inverts, due to increased turgor, the I piece is slipped off and the 

 two cells come apart (cf. fig. 67 C-E). 



When two filaments touch they may form joints or genicula- 

 tions, adhesion being brought about by a mucilaginous secretion 

 produced by the stimulation of the contact. The formation of such 

 geniculations, however, has no connexion with reproduction. 



Sexual reproduction is secured by the process of conjugation, the 

 onset of which is brought about by a combination of certain 

 internal physiological factors combined with the ^H of the external 

 medium. It commonly takes place during the spring phase and 

 then the threads come together in pairs, but either one or more 

 than two filaments may also be involved. The threads first come 

 together by slow movements, the mechanism of which may be 

 connected with the secretion of mucilage; then they become 

 glued together by their mucilage and later young and recently 

 formed cells in both filaments put out papillae. These papillae 

 meet almost immediately, elongate, and push the threads apart. 

 Normally one of the threads produces male gametes and the other 

 female, but occasionally the filaments may contain mixed cells. 

 The papillae from male cells are usually longer and thinner than 

 those from the female cells and so they can fit inside the latter. The 

 conjugating cells accumulate much starch, the nuclei decrease in 

 size and the wall separating the papillae breaks, thus forming a 

 conjugation tube. The whole process so far described forms the 

 maturation phase which is now followed by the phase of gametic 

 union. Contractile vacuoles, which make their appearance in the 

 cytoplasmic lining, remove water from the central vacuole and so 

 cause the protoplasm of the male cells to contract from the walls. 

 The male cytoplasmic mass then migrates through the conjugation 

 tube into the female cell where fusion of the two masses takes place 

 and this is then followed by contraction of the female cytoplasm, 

 though in the larger species it may contract before fusion. Fusion 

 of the two nuclei may be delayed for some time, but in any case the 

 male chloroplasts degenerate. The process described above is 

 known as scalariform conjugation, and it includes certain abnormal 

 cases where cells produce more than one papilla or where the 



